The conventional standards of optical disc media include BD-R, BD-RE, DVD-RAM, DVD-R, DVD-RW, CD-RW, etc. There are techniques of rewriting or incrementally writing data by emitting laser light onto optical disc media that comply with these standards.
An example of the optical disc media is a phase change type optical disc medium. Recording of information on the phase change type optical disc medium is realized by irradiating an optical disc medium with laser light to locally change the state of atomic bond of the material of a thin film formed over a recording film surface by the injected energy of the laser light. The irradiation with the laser light changes the physical state of the irradiated portion and its surrounding portion. Specifically, the crystalline state and the amorphous state have different reflectances. Since a difference in the physical state leads to a difference in reflectance, information can be read out by irradiating the disc with laser light of a sufficiently smaller power than that used in recording and detecting the amount of change in reflectance.
Examples of the phase change type optical disc media include writable media in which a GeSbTe material is used as a recording material for a recording layer as well as write-once optical disc media. Patent Document No. 1 discloses the technique of using a material containing Te—O-M (where M is at least one of metal elements, metalloid elements, and semiconductor elements) as an example of a recording material of a write-once optical disc medium. Te—O-M means a composite material which contains Te, O, and M. Immediately after the formation of a film, particles of Te, Te-M, and M are uniformly and randomly dispersed throughout a matrix of TeO2. Irradiation of a thin film formed of this recording material with converged laser light causes the film to melt, so that crystals of Te or Te-M of large grain size are deposited. The difference of the optical state which is caused in this process can be detected as a signal. This enables the mode of recording in which writing is allowed only once in the same area, so-called write-once recording.
In an alloy-based write-once type disc made of an inorganic material, two thin films made of different materials are combined into a laminate. These materials are heated by laser to melt, so that the materials are mixed together into an alloy, whereby record marks are formed. Another known write-once optical disc medium of a different type is, for example, such that the temperature is increased by laser irradiation to thermally decompose organic pigments of an organic pigment material, and the change in refractive index of the decomposed part is decreased, whereby information is recorded. In the write-once optical disc medium of this type, the principle of recording of information is such that the optical path length of a light transmission layer appears shorter in a recorded portion than in a unrecorded portion, and as a result, this acts like concavity/convexity pits of, for example, a read-only CD on incoming light.
In the case of mark edge recording on such a write-once optical disc medium, the optical disc medium is irradiated with laser light consisting of a plurality of pulse trains called “multi-pulses” such that the physical state of marks is changed, whereby information is recorded. The information is read out by detecting the change in reflectance.
A conceivable measure for increasing the recording density is, typically, to decrease the length of marks and spaces which are to be recorded. However, especially when the length of a space with a preceding record mark becomes shorter, thermal interference occurs such that the heat at the trailing end of the recorded mark is conducted via a space portion to affect the increase in temperature at the leading end of a succeeding mark and, on the other side, the heat at the leading end of the recorded mark affects the cooling cycle at the trailing end of a preceding mark. Even when marks and spaces formed on a track have correct lengths, edge positions of short marks and spaces which are detected in reproduction are disadvantageously different from their ideal values due to the frequency characteristics of a reproduction optical system which depend on the size of the light spot. The deviation of the detected edges from the ideal values is generally referred to as “intersymbol interference”. When the sizes of marks and spaces are smaller than the light spot, large intersymbol interference occurs, and accordingly, the jitter in reproduction is increased, so that the bit error rate is increased.
At the recording densities of DVDs and BDs, the sizes of marks which are to be recorded and the distance between the marks and spaces are small. As a result, the heat of laser light applied for formation of a mark not only reaches an intended area for the mark but also is conducted via spaces to reach the areas for preceding and succeeding marks, so that deformation can sometimes occur in the shapes of the interested mark and the preceding and succeeding marks. There are known techniques capable of avoiding this problem, for example, the technique of changing the leading pulse position of a multi-pulse for forming a mark is changed according to the relationship of the length of the interested mark and the length of the space with the succeeding mark, and the technique of changing the trailing pulse position of a multi-pulse for forming a mark is changed according to the relationship of the length of the interested mark and the length of the space with the preceding mark. These techniques are techniques of recording marks with preliminary corrections made to thermal interference of record marks. This mode of control of the write pulse position is generally referred to as adaptive recording compensation. Patent Document No. 2 discloses such an adaptive recording compensation method.
According to the recording method disclosed in Patent Document No. 2, a writable optical disc medium contains pre-recorded write pulse reference conditions, by which the positional information of write pulses are specified for respective one of the plurality of possible combinations of the length of a mark, the length of the space with the succeeding mark, or the length of the space with the preceding mark. The recording apparatus retrieves the write pulse reference conditions from the optical disc medium to modify currently-effective write pulse reference conditions such that optimum write pulse conditions are obtained.
Specifically, the positional information established for all the combinations of the mark lengths and the space lengths of the space with the succeeding mark included in the write pulse reference conditions, or for all the combinations of the mark lengths and the space lengths of the space with the preceding mark included in the write pulse reference conditions, are used to perform the first test writing in a predetermined track on an optical disc medium. The information recorded in the first test writing is reproduced, and the first jitter is detected in the reproduced signal. And, a change of the first predetermined amount is uniformly added to the positional information for respective ones of all the combinations of the mark lengths and the space lengths included in the write pulse reference conditions. The uniformly-changed positional information is used to perform the second test writing in a predetermined track on the optical disc medium. The information recorded in the second test writing is reproduced, and the second jitter is detected in the reproduced signal. In the last step, the first jitter and the second jitter are compared, and the positional information which is used in the test writing that generated the smaller jitter is selected to obtain the write pulse conditions.
The recording control methods disclosed in Patent Document No. 3, Patent Document No. 4, and Patent Document No. 5 use maximum likelihood decoding methods, rather than utilizing the jitters in the reproduced signal, in order to pre-estimate a signal pattern from a reproduced signal waveform. While comparing the reproduced signal waveform and the estimated signal waveform, the reproduced signal is converted by decoding into decoded data which has a signal path of the maximum likelihood. This method is used to optimize the recording parameters in recording of information such that the probability of occurrence of errors in the process of maximum likelihood decoding is minimized.
In recent years, the higher densities of optical disc media cause the lengths of record marks to come closer to the optical resolution limit, so that increase in intersymbol interference and deterioration in SNR (Signal to Noise Ratio) become larger.
The system margin can be maintained by using a higher-order PRML method. For example, Non-Patent Document No. 1 discloses that, under the circumstances where the optical system is such that the laser wavelength is 405 nm and the NA (Numerical Aperture) of the objective lens is 0.85 and the recording density is such that a Blu-ray Disc (BD) with the diameter of 12 cm has the capacity of 25 GB (Giga Byte) per data recording layer, the system margin can be secured by employing the PR (1, 2, 2, 1) ML method. This document also discloses that, in the case where the linear density is increased by decreasing the mark length in order to secure a storage capacity of 25 GB or larger (e.g., 30 GB or 33.4 GB) per data recording layer while the same optical system is used, it is necessary to employ the PR(1,2,2,2,1)ML method.
Patent Document No. 6, Patent Document No. 7 and Patent Document No. 8 disclose optimizing various recording parameters by adjusting the write pulse waveform based on the quality of composite data in accordance with the PR(1,2,2,2,1)ML method in the case of a high recording density optical disc medium of 30 GB to 33.4 GB per data recording layer.